LTE Handovers

Handovers is one of the major parts of LTE learning curve. I just started reading how handovers work in LTE. This post is to set expectations on how handovers work and give an over all idea. Handovers are defined in 3GPP TS 23.401 (Refer to the latest spec).

Before moving further lets understand the variables here. UE is the one which moving from one place to other. Along with it the network changes and so devices. For this post lets just stick on to LTE, that is we are talking only of Rel 8 LTE network here. As the network changes the possible devices that can change are eNodeB, MME and SGW. I usually consider that SGW and PGW are built in single device so they will always be placed together. Let me put a LTE network architecture here for reference. Too lazy to make a powerpoint slide so pulling one from internet :) (Image source : www.catapult.com)

UE is moving from one place to other connecting to different eNB. In turn eNB's are conne cted to MME and SGW. MME is connected to SGW and eNB's. So when UE move from one area to other the following combinations arise : -

• Inter eNB handover or Intra MME handover : That means UE is moving from one eNB to other where both eNB's are connected to same MME.
• Inter eNB handover with MME change : Here UE is moving from one eNB to other where each of the eNB's is connected to different MME's.
• Inter eNB handover with MME and SGW change : Here UE is moving from one eNB to other where each eNB is connected to different MME and SGW.

The above are my naming conventions, we will go to what spec calls them in a minute. There are two things we need to understand. First is the way connections are made. eNB is connected to MME for control plane signaling and also has a direct link connected to S-GW for user plane traffic. So when there is change in eNB there is possibility that SGW handover has to be performed. Next is the way these devices communicate. Here is where we will have to look at two new protocols S1-AP and X2-AP. S1-AP is used for communication between two MME's and X2-AP is used for communication between two eNB's. GTPv2 is present in S11 and S5/S8 interfaces. eNB may communicate with MME using S1-AP protocol too.

3GPP Spec defines the above three scenarios as:

• X2 Based Handover : without SGW change and with SGW change (3GPP TS 23.401 Clause 5.5.1.1)
• S1 Based Handover : with eNB, MME and SGW change (3GPP TS 23.401 Clause 5.5.1.2)

We need to understand the UE and EPC behavior with respect to default and dedicated bearers. I dont believe that there will be any change in QoS with handovers.

This is brief over view of LTE handovers. In coming days I want to discuss each handover in detail with call flows. Unfortunately I will not be able to dig up S1-AP and X2-AP protocols in details but will do my best. I feel we will jump into few more handover scenarios as we proceed.

Until then adios. Comments are welcome.